Biochemical studies of the last several years have shown that uptake of inorganic iron and its transport in microorganisms takes place using very powerful chelating agents which coordinate to iron via hydroxamate functional groups. The siderochromes are a class of compounds which are trihydroxamic acids and are used by various microorganisms to extract ferric iron from their solution environment and concentrate it within the cells. Since the pathogenicity of microbial infections is apparently due to their disruption of iron transport, it is not surprising that several of the siderochromes are potent and broad spectrum antibiotics, while others are growth factors. Transport of the siderochromes across cell membranes is very conformation dependent; the metal complexes are rapidly passed into the cell while the demetallo forms are not. It is proposed that Cr3 plus (and perhaps Co3 plus ) be substituted for Fe3 plus in the siderochromes. Unlike Fe3 plus, the substituted complexes will be kinetically inert and so the various possible coordination isomers can be individually isolated. We propose to test the conformational specificity of cell transport by comparing the biological activity of the individual coordination isomers of the Cr3 plus siderochrome complexes.